1: Flatten the image2: Resize image to 1.666 times the final size you want. If your final output size is 600px then you should resize at this point to 1000px3: Apply the Sharpen filter (not Unsharp Mask)4: Duplicate the layer5: Apply Sharpen filter (to the top layer, it will look oversharpened)5: Resize to final output size. So if you have just resized your iamge to 1000px resize it to 600px now.6: Use a layer mask on the top (sharpest) layer to adjust opacity and mask areas which look oversharpened.7: You may wish to slightly correct the saturation which will have changed during the process.

BTW I did another way and got even better results as for me. Maybe this will be helpful for you:1. Duplicate the basic layer with image.2. Apply Filter -> Other -> High Pass with 0.5 radius.3. Change the layer blend mode to Overlay.4. Duplicate Overlay layer.5. Add mask to second Overlay layer and adjust opacity and mask areas if oversharp.